Process for the electrodeposition of iron-chromium alloys



PROCESS FOR THE ELECTRODEPOSITION OF lRON-CHROMIUM ALLOYS Tadashi Yoshida, Shinjuku-ku, Tokyo, Japan No Drawing. Application November 9, 1953, Serial No. 391,149

1 Claim. (Cl. 204-43) The present invention relates to a process for the electro-deposition of iron-chromium alloy in which an electrolytic bath is used containing chromic sulfate, ferrous sulfate and free urea as the principal components.

The object of this invention is to provide an established industrial process for the electrodeposition of iron-chromium alloys, beautiful and high corrosion resistant alloys, such as so-called 13% chromium alloy or 18% chromium alloy.

Fuseya et al. (G. Fuseya and K. Sasaki: Chemical News, vol. 141, page 407, 1930, Transactions of the Electrochemical Society, vol. 59, page 445, 1931) have tried to obtain electroposition of iron-chromium alloy by using an electrolytic bath containing chromic sulfate and ferrous sulfate and ferrous sulfate as the principal constituents. This showed merely the possibility thereof and no sufficiently good result could be obtained, as they describe.

As is well known, iron deposits mainly from its divalent state. However, it is almost impossible to hold iron in the divalent state in a chromic acid solution. Consequently, it is also almost impossible to deposit iron-chromium alloy from a chromic acid bath containing iron lons.

It seems to be due to the fact that no process for chromium electrodeposition using a trivalent chromium bath has been reported, which has been successful in industrial practice, or that no industrially usable process for the electrodeposition of iron-chromium alloy has thus far been known, though it has been long desired.

With the object of establishing an industrial process for the electrodeposition of iron-chromium alloy, the present inventor has conducted, in the first place, an investigation on a process for obtaining easily the electrodeposition of metallic chromium of fine quality from an aqueous solution which contains chromic sulfate as a principal constituent and hasmade an invention as described in the specification for U. S. Pat. 2,704,273, issued March 15, 1955.

With respect to the features and advantages of the above mentioned process for the electrodeposition of chromium there will be illustrated briefly as in the following, in which the findings from the subsequent study will be incorporated:

(1) Addition of free urea t the electrolytic bath. When any suitable quantity of free urea is added to the electrolytic bath, said free urea, holding relatively reducing atmosphere in said bath, reduces the spontaneous oxidation of the extremely unstable divalent chromium forming at the cathode during the electrolysis, and simultaneously renders the deposition potential of chromium comparatively noble, and thereby the deposition of metallic chromium is facilitated. Further, the free urea existing in the bath exhibiting a relatively high buffering action, accelerates the tendency of depositing fine metallic chromium at high efiiciency, with the preventing of the formation of basic precipitate of chromium, by alleviating the atmosphere of an extremely low hydrogen ion tates Patent concentration occurring locally in the vicinity of the cathode during the electrolysis.

The present inventor has aflirmed, based upon the experimental grounds, from the determination of the oxidation-reduction potential, observation of the polarogram or the determination of electric conductivity, etc., that not only the above-mentioned action of urea is obtainable but also in this case the urea molecules in the solution are almost existing in free state, without forming complex lOIlS.

This is to say, if, in this case, a major part of urea molecules in the bath should constitute complex ions, the above-mentioned action of urea could not be explained theoretically.

Besides, in the preparation of crystal compound, such as [Cr(Urea)e]2(SO4)s or [Cr(Urea)s]Cl3, especially troublesome procedures are needed; and at the same time the urea molecules is existent forming complex ions [Cr(Urea)e] in the aqueous solutions of those compounds, which is obvious from the references, for instance, E. Wilke-Doerfurt and K. Niederer: Zeitschrift fuer Anorganische Chemie, vol. 184, page 145, 1929. Moreover, it has never been found in any record that a compound of [Cr(Urea)s]2(SO4)3 could be separated from the green solution of chromic sulfate and urea as used by the present inventor in the aforementioned process for the electrodeposition of chromium.

R. W. Parry et al. (R. W. Parry et al.: Transactions of The Electrochemical Society, vol. 92, page 507, 1947) reported from the result of electrolysing an aqueous solution of crystal compound of [Cr(Urea)6]Cl3 that it was almost impossible to precipitate metallic chromium. Considering on their case, a majority of urea molecules in the bath ought to be in a combining state corresponding to [Cr(Urea)s] Therefore, it is certain that the effect as obtained by the urea molecule existing in the free state could not be developed, and consequently no satisfactory result could be obtained. It is evident from the foregoing descriptions that the aforementioned bath for chromium electrodeposition introduced by the present inventor is entirely different from those employed by R. W. Parry et al.

2. Aging of electrolytic bath-There are existent in the majority of cases in the green aqueous solution containing chromic sulfate alone, two or more than two species of chromic complex ions, such as, for instance,

Further, the state of complex ions described above varies gradually with conditions. At a given temperature in this case, aging for a relatively long period of time is necessary before there has been reached an equilibrium of the state of complex ions described above.

it has been found that, only When the aforementioned electrolytic bath, of which complex ions have reached an equilibrium at the temperature of electrolysis, is subjected to an electrolysis, not only reproducible results are obtainable, but also an exceedingly more satisfactory electrodeposition is obtainable, as compared with the case when an unstable complex ion electrolytic bath is used. Namely, the chromic sulfate bath should be kept at a constant electrolytic temperature for a relatively long interval of time, so that the complex ions in the bath as described above may be given time enough to reach an equilibrium for the temperature of electrolysis, before it is used for the plating operation. In this case, however, the complex ions of chromium as referred to the specification of this application denotes complex ions, such as, for instance, [Cr(H2O)e] [Crz (H2O)10SO4] or [Cr2(H2O)s(SO4)2] or [C12(H20)6OH(SO4)2] and does not indicate such complex ions, as those having urea molecules in the complex radical unless otherwise stated.

The present inventor has succeeded in obtaining, the electrodeposition of industrially valuable iron-chromium alloys by using an electrolytic bath as follows of a ratio of trivalent chromium to divalent iron contents of more than 1.3, as considering the relative positions of iron and chromium in the electrochemical series, the iron will deposit remarkably more easily than chromium, in direct utilization of the result of the above-mentioned investigation on the electrodeposition of chromium.

The details of the process for electrodeposition according to this invention are as follows:

THE ELECTROLYTIC BATH As an electrolytic bath is used an aqueous solution, which has reached an equilibrium with respect to the complex ion by being held at a constant electrolytic temperature from 25 to 55 C., having a Weight ratio of the trivalent chromium to divalent iron contained therein of more than 1.3 and containing from 10 to 63 gr. per litre of trivalent chromium, from 5 to 40 gr. per litre of divalent iron, from 100 to 264 gr. per litre of free urea and at least stoichiometrically equivalent amount of sulfate radical :to the total of the trivalent chromium and divalent iron contained.

From the industrial standpoint, however, it is convenient that an adequate amount either of ammonium sulfate, sodium sulfate or potassium sulfate is added to the aforementioned electrolytic bath to improve the conductivity and a suitable amount of chlorine ion is also added therein to obtain an optimum solution velocity of anode.

Here, the time necessary for the aforementioned electrolytic bath to attain an equilibrium with respect to the aforementioned complex ions at constant temperatures is respectively about 270 hours at 25 C., about 90 hours at 35 (1., about 20 hours at 45 C. and about 8 hours at 55 C. Those necessary durations of time may be obtained in pursuit of the conductivity variation of the electrolytic bath at each respective constant temperatures.

Further, when the Weight ratio of trivalent chromium to divalent iron contained in the electrolytic bath is not more than 1.3, or either of the contents of those constituents is not within the above mentioned range, it is difiicult to obtain a deposition of a satisfactorily good alloy.

ANODE An anode of iron-chromium alloy having the same composition as that of the electrodeposit is preferably used.

In this case, commercial alloys such as those ironchromium alloys containing 13% or 18% chromium may likewise be used as an anode.

CATHODE An electric conductive material on which electrodeposition is to be made or such to be galvanized may be used as a cathode.

ELECTROLYSIS Keeping the above-mentioned electrolytic bath at a constant temperature between 25 and 55 C., the electrolysis is carried out at the cathode current density of 7 to 30 amperes per drn. It is thereby possible to obtain Example 1 Electrolytic bath-An aqueous solution containing per liter, gr. of Crz(SO4)s, 60 gr. of FeSO4, 200 gr. of (NH4}2SO4, gr. of free (NH2)2CO and 10 gr. of NHCi was prepared, which solution was used in the electrolytic operation after having been kept for about 50 hours at 40 C.

An0de.A rolled sheet of commercial iron-chromium alloy containing about 18% chromium was used.

Cr1tl10dc.-A pure copper sheet was used as a cathode.

EIectrolysis.The above-mentioned electrolytic bath was maintained at 40 C. and the electrolysis was carried out at the cathode current density of approximately 15 ainperes per dm while the bath was stirred at a suitable speed. At the end of a 20 minute operation, a fine coating of iron-chromium alloy containing about 21% chromium of about 0.01 mm. thickness was obtained.

Example 2 With otherwise the same electrolytic bath as in Example 1 but containing neither (NH4)2SO4 nor NHCl, a substantially equal good result may be obtainable as in Example 1. In this case, however, the conductivity of the electrolytic bath is too low and the solution velocity of the anode is also too slow, which renders it remarkably disadvantageous from the viewpoint of industrial practice.

Although specific embodiments of the invention have been described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of the invention as defined in the appended claim.

What is claimed:

An electrolytic process for the electro-deposition of an iron-chromium alloy wherein an aqueous solution is used which contains per liter from 10 to 63 grams of trivalent chromium, from 5 to 40 grams of divalent iron, from 100 to 264 grams of free urea and an amount of sulfate radical at least stoichiometrically equivalent to the total content of trivalent chromium and divalent iron contained in said solution, said trivalent chromium having a weight ratio to said divalent iron of more than 1.3, the molecules of said free urea existing in said solution substantially without formation of complex ions therefrom, said solution having attained an equilibrium with respect to complex ions contained therein by havng been kept at a constant temperature between 25 and 55 C.

Fuseya et al.: Transactions Electrochemical Society, vol. 59 (1931), pp. 445-460.

Parry et al.: Transactions Electrochemical Society, vol. 92 (1947), pp. 507-518. 

